1. Field of the Invention
The present invention relates to a signal processing circuit that detects capacitance of a measured capacitor.
2. Description of the Related Art
FIG. 5 illustrates a general circuit diagram of a CV amplifier for a capacitance sensor according to the related art. In such a CV amplifier, a capacitance value of a capacitance sensor element varying depending on humidity or acceleration is changed, and a difference between this capacitance value and a reference capacitance value is detected by an IC. Output noise of such a CV amplifier is determined, for example, based on an average value of an output of an operational amplifier 116. Since an average consumption current is determined based on a period of time in which a circuit operates, performance is determined based on sampling operation speed. A voltage of about half of a power supply voltage is usually necessary for a positive input terminal (+) of the operational amplifier 116 of a CV amplifier block due to a limited operation range of the operational amplifier 116. Usually, this voltage is supplied using a voltage of an output terminal of the operational amplifier 116, as illustrated in FIG. 5.
However, when an operation speed of the operational amplifier 116 of the CV amplifier increases, a capacitor C103 on a positive feedback path from the output terminal of the operational amplifier 116 to the positive input terminal (+) causes oscillation since an initial stage of an ADC includes capacitive sampling of the capacitor C103 based on a reference voltage VREF. This is because an input impedance of a place at which the reference voltage VREF is supplied from the outside is low, but is not 0, and thus, the reference voltage VREF varies according to increase or decrease in a line voltage of the positive feedback path. Therefore, the CV amplifier is likely to oscillate due to the positive feedback path including the capacitor C103.